Method of decoding a pattern-encoded coordinate

ABSTRACT

A method of decoding a coding pattern disposed on or in a substrate. The method comprises the steps of: (a) operatively positioning an optical reader relative to a surface of the substrate; (b) capturing an image of a portion of the coding pattern, the coding pattern comprising a plurality of square tags of length/identifying two-dimensional location coordinates; and (c) sampling and decoding x-coordinate data symbols within the imaged portion and y-coordinate data symbols within the imaged portion. The imaged portion has a predetermined diameter and is guaranteed to contain sufficient data symbols from each of the Reed-Solomon codes so that symbol errors are correctable in each of the codes during the decoding.

FIELD OF INVENTION

The present invention relates to a position-coding pattern on a surface.

COPENDING APPLICATIONS

The following applications have been filed by the Applicantsimultaneously with the present application:

NPT118US NPT119US NPT120US NPT121US NPT122US NPT123US NPT124US NPT125USNPT127US NPT128US NPT129US

The disclosures of these co-pending applications are incorporated hereinby reference. The above applications have been identified by theirfiling docket number, which will be substituted with the correspondingapplication number, once assigned.

CROSS REFERENCES

The following patents or patent applications filed by the applicant orassignee of the present invention are hereby incorporated bycross-reference.

10/815,621 10/815,635 10/815,647 11/488,162 10/815,636 11/041,65211/041,609 11/041,556 10/815,609 7,204,941 7,278,727 10/913,3807,122,076 7,156,289 09/575,197 6,720,985 7,295,839 09/722,174 7,068,3827,094,910 7,062,651 6,644,642 6,549,935 6,987,573 6,727,996 6,760,1197,064,851 6,290,349 6,428,155 6,785,016 6,831,682 6,741,871 6,965,43910/932,044 6,870,966 6,474,888 6,724,374 6,788,982 7,263,270 6,788,2936,737,591 09/693,514 10/778,056 10/778,061 11/193,482 7,055,7396,830,196 7,182,247 7,082,562 10/409,864 7,108,192 10/492,169 10/492,15210/492,168 10/492,161 7,308,148 6,957,768 7,170,499 11/856,06111/672,522 11/672,950 11/754,310 12/015,507 7,148,345 12/025,74612/025,762 12/025,765 10/407,212 6,902,255 6,755,509 12/178,61112/178,619

BACKGROUND

The Applicant has previously described a method of enabling users toaccess information from a computer system via a printed substrate e.g.paper. The substrate has a coding pattern printed thereon, which is readby an optical sensing device when the user interacts with the substrateusing the sensing device. A computer receives interaction data from thesensing device and uses this data to determine what action is beingrequested by the user. For example, a user may make handwritten inputonto a form or make a selection gesture around a printed item. Thisinput is interpreted by the computer system with reference to a pagedescription corresponding to the printed substrate.

It would desirable to improve the coding pattern on the substrate so asto maximize the data capacity of the coding pattern and minimize theoverall visibility of the coding pattern on the substrate. The codingpattern typically comprises data symbols (encoding ‘useful’ information,such as an identity and/or a location) and as well as other symbols,which allow the data symbols to be decoded. It would be desirable tominimize the amount of data encoded in these other symbols so as to makemaximum use of the data symbols. By minimizing the space occupied bythese other symbols, there is more space available in the coding patternfor data symbols which encode useful information.

It would be further desirable to distribute coordinate data symbols soas to maximize the error-correcting capacity of the code. In particular,it would be desirable to provide maximum robustness even when, forexample, the coding pattern is scratched with horizontal or verticallines (e.g. if the coding pattern is co-printed with form field boxescontaining horizontal and vertical lines, which potentially affect thereadability of the coding pattern).

SUMMARY OF INVENTION

In a first aspect, there is provided a substrate having a coding patterndisposed thereon or therein, said coding pattern comprising a pluralityof square tags of length l identifying two-dimensional locationcoordinates, each tag comprising:

a plurality n₁ of x-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for an x-coordinate;

a plurality n₁ of y-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for a y-coordinate;

a number z of said x-coordinate data symbols positioned in a centralcolumn of width q in each tag with a remaining (n₁−z) x-coordinate datasymbols distributed evenly on either side of said central column; and

a number z of said y-coordinate data symbols positioned in a central rowof said tag of width q with a remaining (n₁−z) y-coordinate data symbolsdistributed evenly above and below said central row;

wherein n₁≧n₂ and z≧1, such that any square portion of said codingpattern of length (l+q) is guaranteed to contain at least (n₁+z)/2 datasymbols from each of said Reed-Solomon codes.

The distribution of coordinate data symbols in the coding patterndefined in connection with the first aspect provides excellentrobustness in the presence of errors, particularly errors resulting fromdefects in horizontal or vertical lines in the coding pattern.

Optionally, n₁=n₂ and none of said x-coordinate or y-coordinate datasymbols is replicated within each tag.

Optionally, n₂>2k.

Optionally, z≧k.

Optionally, each of said Reed-Solomon codes is a natural full-lengthcode or a shortened code.

Optionally, z is from 1 to 253, optionally from 1 to 15 or optionallyfrom 2 to 7, or optionally from 2 to 5.

Optionally, k is from 1 to 253; optionally from 1 to 15 or optionallyfrom 2 to 9, or optionally from 2 to 5. Optionally z=k±2, z=k±1 or z=k.

Optionally, n₁ is from 3 to 255, optionally from 3 to 31, optionallyfrom 3 to 15, or optionally from 3 to 9.

Optionally, n₂ is from 2 to 255, optionally from 3 to 31, optionallyfrom 3 to 15, or optionally from 3 to 9.

Optionally, up to 126 symbol errors are corrected during said decoding.Optionally, at least 1, at least 2, at least 3, at least 4, at least 5,at least 6, at least 7, at least 8 or at least 9 symbol errors arecorrected during said decoding.

Optionally, each data symbol is encoded using multi-pulse positionmodulation.

Optionally, each data symbol is represented by d macrodots, each of saidd macrodots occupying a respective position from a plurality ofpredetermined possible positions p, the respective positions of said dmacrodots representing one of a plurality of possible data values,wherein p>d.

Optionally, d is an integer value of 2 to 6 and p is an integer value of4 to 12. Typically, p is 7 and d is either 2 or 3, corresponding to a2-7 PPM or 3-7 PPM encoding of data symbols.

Optionally, each data symbol provides i possible symbol values, andwherein any unused symbol values are treated as erasures.

Optionally, each of data symbol provides i possible symbol values for aj-bit symbol, wherein (i−2^(j)) unused symbol values are treated aserasures.

Optionally, d=3 and p=7 which provides 35 possible symbol values for a5-bit data symbol, wherein 3 unused symbol values are treated aserasures.

Optionally, d=2 and p=7 which provides 21 possible symbol values for a4-bit data symbol, wherein 5 unused symbol values are treated aserasures.

Optionally, q=2 s, 3 s, 4 s or 5 s, wherein s is defined as a spacingbetween adjacent macrodots.

Optionally, said coding pattern comprises a plurality of target elementsdefining a target grid, said target grid comprising a plurality of gridcells, wherein neighboring grid cells share target elements and whereineach tag is defined by a plurality of contiguous grid cells.

Optionally, each tag comprises M² contiguous square grid cells, whereinM is an integer. Optionally, M is an integer having a value of 2, 3, 4,5 or 6.

Optionally, each grid cell defines a symbol group containing a pluralityof data symbols, wherein each symbol group comprises at least one ofsaid x-coordinate symbols and/or at least one of said y-coordinatesymbols.

Optionally, each tag comprises one or more common Reed-Solomoncodewords, each common codeword being common to a plurality ofcontiguous tags, each common codeword comprising a respective set ofcommon data symbols.

Optionally, each common codeword, or a set of common codewords,identifies an identity. The identity typically identifies at least oneof: the substrate; a region; a page; a product; a visual layout; and aninteractivity layout.

Optionally, each symbol group comprises one or more registrationsymbols, said registration symbols identifying one or more of:

a tag encoding format;

a translation of a symbol group relative to a tag containing said symbolgroup;

an orientation of a layout of said data symbols with respect to saidtarget grid; and

a flag.

Optionally, said tag encoding format identifies a number of macrodotscontained in each data symbol of said coding pattern.

In a second aspect, there is provided a method of decoding a codingpattern disposed on or in a substrate, said method comprising the stepsof:

(a) operatively positioning an optical reader relative to a surface ofsaid substrate;

(b) capturing an image of a portion of said coding pattern, said codingpattern comprising a plurality of square tags of length/identifyingtwo-dimensional location coordinates, each tag comprising:

a plurality n₁ of x-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for an x-coordinate;

a plurality n₁ of y-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for a y-coordinate;

a number z of said x-coordinate data symbols positioned in a centralcolumn of width q in each tag with a remaining (n₁−z) x-coordinate datasymbols distributed evenly on either side of said central column; and

a number z of said y-coordinate data symbols positioned in a central rowof said tag of width q with a remaining (n₁−z) y-coordinate data symbolsdistributed evenly above and below said central row, wherein n₁≧n₂ andz≧1;

(c) sampling and decoding at least one of:

-   -   x-coordinate data symbols within said imaged portion; and    -   y-coordinate data symbols within said imaged portion,        wherein said imaged portion has a diameter of at least (l+q)√2        and less than (2l)√2 and is guaranteed to contain at least        (n₁+z)/2 data symbols from each of said Reed-Solomon codes, and        wherein one or more symbol errors are correctable in each of        said codes during said decoding.

Optionally, said imaged portion has a diameter of at least (l+q)√2 andless than (2l)√2 and is guaranteed to contain at least (n₁+z)/2coordinate data symbols from each of said local Reed-Solomon codes.

Optionally, said sampled x-coordinate data symbols and y-coordinate datasymbols within said imaged portion define respective sampledx-coordinate and y-coordinate codewords, and wherein any missingcoordinate data symbols from respective (n₂, k) Reed-Solomon codes aretreated as erasures in said sampled codewords.

Optionally, a number of said missing coordinate data symbols fromrespective (n₂, k) Reed-Solomon codes varies from 0 to (n₁−z)/2depending on an alignment of said imaged portion with a tag to bedecoded.

Optionally, each tag comprises one or more common Reed-Solomon codewordsidentifying a region identity associated with said surface, each commoncodeword being common to a plurality of contiguous tags and each commoncodeword comprising a respective set of common data symbols, whereinsaid method comprises the further steps of:

sampling and decoding common data symbols within said imaged portion;and

determining a region identity using said decoded common data symbols

Optionally, each symbol group comprises one or more registrationsymbols, said registration symbols identifying one or more of:

a tag encoding format;

a translation of a symbol group relative to a tag containing said symbolgroup;

an orientation of a layout of said data symbols with respect to saidtarget grid; and

a flag,

wherein said method comprises the further step of:

sampling and decoding registration symbols within the imaged portion;and

using the decoded registration symbols to decode the x-coordinate datasymbols and y-coordinate data symbols

Optionally, said method comprises the further step of:

-   -   using the decoded registration symbols to determine a tag        encoding format for said coding pattern, said tag encoding        format identifying a number of macrodots contained in each of        said data symbols.

Optionally, said method comprises the further step of:

-   -   using the decoded registration symbols to determine whether at        least one tag within said imaged portion is flagged or        unflagged; and    -   providing feedback to a user in the event that the tag is        flagged.

In a third aspect, there is provided a system for decoding a codingpattern, said system comprising:

(A) a substrate having a coding pattern disposed thereon or therein,said coding pattern comprising a plurality of square tags oflength/identifying two-dimensional location coordinates, each tagcomprising:

a plurality n₁ of x-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for an x-coordinate;

a plurality n₁ of y-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for a y-coordinate;

a number z of said x-coordinate data symbols positioned in a centralcolumn of width q in each tag with a remaining (n₁−z) x-coordinate datasymbols distributed evenly on either side of said central column; and

a number z of said y-coordinate data symbols positioned in a central rowof said tag of width q with a remaining (n₁−z) y-coordinate data symbolsdistributed evenly above and below said central row, wherein n₁≧n₂ andz≧k; and

(B) an optical reader comprising:

an image sensor for capturing an image of a portion of said codingpattern, said image sensor having a field of view with a diameter of atleast (l+q)√2 and less than (2l)√2; and

a processor configured for performing the steps of:

-   -   (i) sampling at least (n₁+z)/2 data symbols from at least one of        said Reed-Solomon codes; and    -   (ii) decoding at least one of: said x-coordinate data symbols        within said imaged portion; and said y-coordinate data symbols        within said imaged portion,        wherein said processor is configured to correct one or more        symbol errors in each of said codes when decoding said        x-coordinate and y-coordinate data symbols.

In a fourth aspect, there is provided an optical reader for decoding acoding pattern disposed on or in a substrate, said coding patterncomprising a plurality of square tags of length l identifyingtwo-dimensional location coordinates, each tag comprising:

a plurality n₁ of x-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for an x-coordinate;

a plurality n₁ of y-coordinate data symbols encoding a respective (n₂,k) Reed-Solomon code for a y-coordinate;

a number z of said x-coordinate data symbols positioned in a centralcolumn of width q in each tag with a remaining (n₁−z) x-coordinate datasymbols distributed evenly on either side of said central column; and

a number z of said y-coordinate data symbols positioned in a central rowof said tag of width q with a remaining (n₁−z) y-coordinate data symbolsdistributed evenly above and below said central row, wherein n₁≧n₂ andz≧k;

said optical reader comprising:

an image sensor for capturing an image of a portion of said codingpattern, said image sensor having a field of view with a diameter of atleast (l+q)√2 and less than (2l)√2; and

a processor configured for performing the steps of:

-   -   (i) sampling at least (n₁+z)/2 data symbols from at least one of        said Reed-Solomon codes; and    -   (ii) decoding at least one of: said x-coordinate data symbols        within said imaged portion; and said y-coordinate data symbols        within said imaged portion,        wherein said processor is configured to correct one or more        symbol errors in each of said codes when decoding said        x-coordinate and y-coordinate data symbols.

In a fifth aspect, there is provided a method of decoding a codingpattern disposed on or in a substrate, said method comprising the stepsof:

(a) operatively positioning an optical reader relative to a surface ofsaid substrate;

(b) capturing an image of a portion of said coding pattern, said codingpattern comprising a plurality of tags, each tag comprising:

a plurality n₁ of local data symbols encoding a local (n₂, k)Reed-Solomon code for said tag, wherein n₁≧n₂;

(c) sampling a plurality of said local data symbols within said imagedportion, said plurality of sampled local data symbols defining at leastpart of a sampled local codeword;

(d) identifying n₃ missing data symbols in said sampled local codeword,said missing data symbols being positioned at least partially outsidesaid imaged portion; and

(e) decoding said sampled local codeword by treating said n₃ missingdata symbols as erasures,

wherein n₃ varies depending on an alignment of said imaged portion witha tag to be decoded.

Optionally, only part of said tag to be decoded is contained within saidimaged portion and n₃ is at least 1.

Optionally, 0, 1, 2, 3, 4 or 5 symbol errors are corrected during saiddecoding.

Optionally, n₁=n₂ and none of said local data symbols is replicatedwithin each tag.

Optionally, said n₃ missing data symbols are identified using thealignment of the imaged portion with the tag to be decoded.

Optionally, n₃ varies between 0 and n_(max) for all possible alignments,and wherein n_(max) is determined by a diameter of said imaged portionand an arrangement of said local data symbols in each tag.

Optionally, (n₂−n_(max))>k.

Optionally, n_(max) is from 2 to 15.

Optionally, each tag is a square tag of length l comprising:

a plurality n₁ of local x-coordinate data symbols encoding a respectivelocal (n₂, k) Reed-Solomon code for an x-coordinate;

a plurality n₁ of local y-coordinate data symbols encoding a respectivelocal (n₂, k) Reed-Solomon code for a y-coordinate;

a number z of said x-coordinate data symbols positioned in a centralcolumn of width q in each tag with a remaining (n₁−z) x-coordinate datasymbols distributed evenly on either side of said central column; and

a number z of said y-coordinate data symbols positioned in a central rowof said tag of width q with a remaining (n₁−z) y-coordinate data symbolsdistributed evenly above and below said central row, wherein n₁≧n₂ andz≧k.

Optionally, said imaged portion has a diameter of at least (l+q)√2 andless than (2l)√2 and is guaranteed to contain at least (n₁+z)/2coordinate data symbols from each of said local Reed-Solomon codes.

Optionally, n_(max)=(n₁−z)/2.

In a sixth aspect, there is provided a system for decoding a codingpattern, said system comprising:

(A) a substrate having the coding pattern disposed thereon or therein,said coding pattern comprising a plurality of tags, each tag comprising:

a plurality n₁ of local data symbols encoding a local (n₂, k)Reed-Solomon code for said tag, wherein n₁≧n₂; and

(B) an optical reader comprising:

an image sensor for capturing an image of a portion of said codingpattern; and

a processor configured for performing the steps of:

-   -   (i) sampling a plurality of said local data symbols within said        imaged portion, said plurality of sampled local data symbols        defining at least part of a sampled local codeword;    -   (ii) identifying n₃ missing data symbols in said sampled local        codeword, said missing data symbols being positioned at least        partially outside said imaged portion; and    -   (iii) decoding said sampled local codeword by treating said n₃        missing data symbols as erasures,        wherein n₃ varies depending on an alignment of said imaged        portion with a tag to be decoded.

In a seventh aspect, there is provided an optical reader for decoding acoding pattern disposed on or in a substrate, said coding patterncomprising a plurality of tags, each tag comprising:

a plurality n₁ of local data symbols encoding a local (n₂, k)Reed-Solomon code for said tag, wherein n₁≧n₂;

said optical reader comprising:

an image sensor for capturing an image of a portion of said codingpattern; and

a processor configured for performing the steps of:

-   -   (i) sampling a plurality of said local data symbols within said        imaged portion, said plurality of sampled local data symbols        defining at least part of a sampled local codeword;    -   (ii) identifying n₃ missing data symbols in said sampled local        codeword, said missing data symbols being positioned at least        partially outside said imaged portion; and    -   (iii) decoding said sampled local codeword by treating said n₃        missing data symbols as erasures,        wherein n₃ varies depending on an alignment of said imaged        portion with a tag to be decoded.

It will appreciated that one or more of the optional embodimentsdescribed herein may be equally applicable to any of the first, second,third, fourth, fifth, sixth or seventh aspects.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and other embodiments of the invention will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a layout of coordinate codewords X and Y, with codeword Xshown in bold outline;

FIG. 2 is a schematic of a the relationship between a sample printednetpage and its online page description;

FIG. 3 shows an embodiment of basic netpage architecture with variousalternatives for the relay device;

FIG. 4 shows the structure of a tag;

FIG. 5 shows a symbol group;

FIG. 6 shows the layout of a first 7 PPM data symbol;

FIG. 7 shows the layout of a second 7 PPM data symbol;

FIG. 8 shows the spacing of macrodot positions;

FIG. 9 shows inter-codeword minimum distances in a registration codethat encodes just registration;

FIG. 10 shows inter-codeword minimum distances in a registration codethat encodes registration and two tag format values;

FIG. 11 shows inter-codeword minimum distances in a registration codethat encodes registration, two tag format values and two flag values;

FIG. 12 shows the layout of a registration symbol;

FIG. 13 shows the layout of registration symbols within a symbol group;

FIG. 14 shows the layout of common codewords A, B and C and D;

FIG. 15 shows the layout of a complete tag;

FIG. 16 shows the layout of a Reed-Solomon codeword;

FIG. 17 is a flowchart of image processing;

FIG. 18 shows a nib and elevation of the pen held by a user;

FIG. 19 shows the pen held by a user at a typical incline to a writingsurface;

FIG. 20 is a lateral cross section through the pen;

FIG. 21A is a bottom and nib end partial perspective of the pen;

FIG. 21B is a bottom and nib end partial perspective with the fields ofillumination and field of view of the sensor window shown in dottedoutline;

FIG. 22 is a longitudinal cross section of the pen;

FIG. 23A is a partial longitudinal cross section of the nib and barrelmolding;

FIG. 23B is a partial longitudinal cross section of the IR LED's and thebarrel molding;

FIG. 24 is a ray trace of the pen optics adjacent a sketch of the inkcartridge;

FIG. 25 is a side elevation of the lens;

FIG. 26 is a side elevation of the nib and the field of view of theoptical sensor; and

FIG. 27 is a block diagram of the pen electronics.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

1.1 Netpage System Architecture

In a preferred embodiment, the invention is configured to work with thenetpage networked computer system, a detailed overview of which follows.It will be appreciated that not every implementation will necessarilyembody all or even most of the specific details and extensions discussedbelow in relation to the basic system. However, the system is describedin its most complete form to reduce the need for external reference whenattempting to understand the context in which the preferred embodimentsand aspects of the present invention operate.

In brief summary, the preferred form of the netpage system employs acomputer interface in the form of a mapped surface, that is, a physicalsurface which contains references to a map of the surface maintained ina computer system. The map references can be queried by an appropriatesensing device. Depending upon the specific implementation, the mapreferences may be encoded visibly or invisibly, and defined in such away that a local query on the mapped surface yields an unambiguous mapreference both within the map and among different maps. The computersystem can contain information about features on the mapped surface, andsuch information can be retrieved based on map references supplied by asensing device used with the mapped surface. The information thusretrieved can take the form of actions which are initiated by thecomputer system on behalf of the operator in response to the operator'sinteraction with the surface features.

In its preferred form, the netpage system relies on the production of,and human interaction with, netpages. These are pages of text, graphicsand images printed on ordinary paper, but which work like interactivewebpages. Information is encoded on each page using ink which issubstantially invisible to the unaided human eye. The ink, however, andthereby the coded data, can be sensed by an optically imaging sensingdevice and transmitted to the netpage system. The sensing device maytake the form of a clicker (for clicking on a specific position on asurface), a pointer having a stylus (for pointing or gesturing on asurface using pointer strokes), or a pen having a marking nib (formarking a surface with ink when pointing, gesturing or writing on thesurface). References herein to “pen” or “netpage pen” are provided byway of example only. It will, of course, be appreciated that the pen maytake the form of any of the sensing devices described above.

In one embodiment, active buttons and hyperlinks on each page can beclicked with the sensing device to request information from the networkor to signal preferences to a network server. In one embodiment, textwritten by hand on a netpage is automatically recognized and convertedto computer text in the netpage system, allowing forms to be filled in.In other embodiments, signatures recorded on a netpage are automaticallyverified, allowing e-commerce transactions to be securely authorized. Inother embodiments, text on a netpage may be clicked or gestured toinitiate a search based on keywords indicated by the user.

As illustrated in FIG. 2, a printed netpage 1 can represent ainteractive form which can be filled in by the user both physically, onthe printed page, and “electronically”, via communication between thepen and the netpage system. The example shows a “Request” formcontaining name and address fields and a submit button. The netpage 1consists of graphic data 2, printed using visible ink, and a surfacecoding pattern 3 superimposed with the graphic data. The surface codingpattern 3 comprises a collection of tags 4. One such tag 4 is shown inthe shaded region of FIG. 2, although it will be appreciated thatcontiguous tags 4, defined by the coding pattern 3, are densely tiledover the whole netpage 1.

The corresponding page description 5, stored on the netpage network,describes the individual elements of the netpage. In particular itdescribes the type and spatial extent (zone) of each interactive element(i.e. text field or button in the example), to allow the netpage systemto correctly interpret input via the netpage. The submit button 6, forexample, has a zone 7 which corresponds to the spatial extent of thecorresponding graphic 8.

As illustrated in FIG. 2, a netpage sensing device 400, such as the pendescribed in Section 3, works in conjunction with a netpage relay device601, which is an Internet-connected device for home, office or mobileuse. The pen 400 is wireless and communicates securely with the netpagerelay device 601 via a short-range radio link 9. In an alternativeembodiment, the netpage pen 400 utilises a wired connection, such as aUSB or other serial connection, to the relay device 601.

The relay device 601 performs the basic function of relaying interactiondata to a page server 10, which interprets the interaction data. Asshown in FIG. 2, the relay device 601 may, for example, take the form ofa personal computer 601 a, a netpage printer 601 b or some other relay601 c (e.g. personal computer or mobile phone incorporating a webbrowser).

The netpage printer 601 b is able to deliver, periodically or on demand,personalized newspapers, magazines, catalogs, brochures and otherpublications, all printed at high quality as interactive netpages.Unlike a personal computer, the netpage printer is an appliance whichcan be, for example, wall-mounted adjacent to an area where the morningnews is first consumed, such as in a user's kitchen, near a breakfasttable, or near the household's point of departure for the day. It alsocomes in tabletop, desktop, portable and miniature versions. Netpagesprinted on-demand at their point of consumption combine the ease-of-useof paper with the timeliness and interactivity of an interactive medium.

Alternatively, the netpage relay device 601 may be a portable device,such as a mobile phone or PDA, a laptop or desktop computer, or aninformation appliance connected to a shared display, such as a TV. Ifthe relay device 601 is not a netpage printer 601 b which printsnetpages digitally and on demand, the netpages may be printed bytraditional analog printing presses, using such techniques as offsetlithography, flexography, screen printing, relief printing androtogravure, as well as by digital printing presses, using techniquessuch as drop-on-demand inkjet, continuous inkjet, dye transfer, andlaser printing.

As shown in FIG. 2, the netpage sensing device 400 interacts with aportion of the tag pattern on a printed netpage 1, or other printedsubstrate such as a label of a product item 251, and communicates, via ashort-range radio link 9, the interaction to the relay device 601. Therelay 601 sends corresponding interaction data to the relevant netpagepage server 10 for interpretation. Raw data received from the sensingdevice 400 may be relayed directly to the page server 10 as interactiondata. Alternatively, the interaction data may be encoded in the form ofan interaction URI and transmitted to the page server 10 via a user'sweb browser 601 c. The web browser 601 c may then receive a URI from thepage server 10 and access a webpage via a webserver 201. In somecircumstances, the page server 10 may access application computersoftware running on a netpage application server 13.

The netpage relay device 601 can be configured to support any number ofsensing devices, and a sensing device can work with any number ofnetpage relays. In the preferred implementation, each netpage sensingdevice 400 has a unique identifier. This allows each user to maintain adistinct profile with respect to a netpage page server 10 or applicationserver 13.

Digital, on-demand delivery of netpages 1 may be performed by thenetpage printer 601 b, which exploits the growing availability ofbroadband Internet access. Netpage publication servers 14 on the netpagenetwork are configured to deliver print-quality publications to netpageprinters. Periodical publications are delivered automatically tosubscribing netpage printers via pointcasting and multicasting Internetprotocols. Personalized publications are filtered and formattedaccording to individual user profiles.

A netpage pen may be registered with a netpage registration server 11and linked to one or more payment card accounts. This allows e-commercepayments to be securely authorized using the netpage pen. The netpageregistration server compares the signature captured by the netpage penwith a previously registered signature, allowing it to authenticate theuser's identity to an e-commerce server. Other biometrics can also beused to verify identity. One version of the netpage pen includesfingerprint scanning, verified in a similar way by the netpageregistration server.

1.2 Netpages

Netpages are the foundation on which a netpage network is built. Theyprovide a paper-based user interface to published information andinteractive services.

As shown in FIG. 2, a netpage consists of a printed page (or othersurface region) invisibly tagged with references to an onlinedescription 5 of the page. The online page description 5 is maintainedpersistently by the netpage page server 10. The page descriptiondescribes the visible layout and content of the page, including text,graphics and images. It also describes the input elements on the page,including buttons, hyperlinks, and input fields. A netpage allowsmarkings made with a netpage pen on its surface to be simultaneouslycaptured and processed by the netpage system.

Multiple netpages (for example, those printed by analog printingpresses) can share the same page description. However, to allow inputthrough otherwise identical pages to be distinguished, each netpage maybe assigned a unique page identifier. This page ID has sufficientprecision to distinguish between a very large number of netpages.

Each reference to the page description 5 is repeatedly encoded in thenetpage pattern. Each tag (and/or a collection of contiguous tags)identifies the unique page on which it appears, and thereby indirectlyidentifies the page description 5. Each tag also identifies its ownposition on the page. Characteristics of the tags are described in moredetail below.

Tags are typically printed in infrared-absorptive ink on any substratewhich is infrared-reflective, such as ordinary paper, or in infraredfluorescing ink. Near-infrared wavelengths are invisible to the humaneye but are easily sensed by a solid-state image sensor with anappropriate filter.

A tag is sensed by a 2D area image sensor in the netpage sensing device,and the tag data is transmitted to the netpage system via the nearestnetpage relay device 601. The pen 400 is wireless and communicates withthe netpage relay device 601 via a short-range radio link. It isimportant that the pen recognize the page ID and position on everyinteraction with the page, since the interaction is stateless. Tags areerror-correctably encoded to make them partially tolerant to surfacedamage.

The netpage page server 10 maintains a unique page instance for eachunique printed netpage, allowing it to maintain a distinct set ofuser-supplied values for input fields in the page description 5 for eachprinted netpage 1.

2 Netpage Tags

2.1 Tag Data Content

Each tag 4 identifies an absolute location of that tag within a regionof a substrate.

Each interaction with a netpage should also provide a region identitytogether with the tag location. In a preferred embodiment, the region towhich a tag refers coincides with an entire page, and the region ID istherefore synonymous with the page ID of the page on which the tagappears. In other embodiments, the region to which a tag refers can bean arbitrary subregion of a page or other surface. For example, it cancoincide with the zone of an interactive element, in which case theregion ID can directly identify the interactive element.

As described in the Applicant's previous applications (e.g. U.S. Pat.No. 6,832,717), the region identity may be encoded discretely in eachtag 4. The region identity may be encoded by a plurality of contiguoustags in such a way that every interaction with the substrate stillidentifies the region identity, even if a whole tag is not in the fieldof view of the sensing device.

Each tag 4 should preferably identify an orientation of the tag relativeto the substrate on which the tag is printed. Orientation data read froma tag enables the rotation (yaw) of the pen 101 relative to thesubstrate to be determined

A tag 4 may also encode one or more flags which relate to the region asa whole or to an individual tag. One or more flag bits may, for example,signal a sensing device to provide feedback indicative of a functionassociated with the immediate area of the tag, without the sensingdevice having to refer to a description of the region. A netpage penmay, for example, illuminate an “active area” LED when in the zone of ahyperlink.

A tag 4 may also encode a digital signature or a fragment thereof. Tagsencoding (partial) digital signatures are useful in applications whereit is required to verify a product's authenticity. Such applications aredescribed in, for example, US Publication No. 2007/0108285, the contentsof which is herein incorporated by reference. The digital signature maybe encoded in such a way that it can be retrieved from every interactionwith the substrate. Alternatively, the digital signature may be encodedin such a way that it can be assembled from a random or partial scan ofthe substrate.

It will, of course, be appreciated that other types of information (e.g.tag size etc) may also be encoded into each tag or a plurality of tags.

2.2 General Tag Structure

As described above in connection with FIG. 2, the netpage surface codinggenerally consists of a dense planar tiling of tags. In the presentinvention, each tag 4 is defined by a coding pattern which contains twokinds of elements. Referring to FIGS. 3 and 4, the first kind of elementis a target element. Target elements in the form of target dots 301allow a tag 4 to be located in an image of a coded surface, and allowthe perspective distortion of the tag to be inferred. The second kind ofelement is a data element in the form of a dot or macrodot 302 (see FIG.7). Collections of the macrodots 302 encode data values. As described inthe Applicant's earlier disclosures (e.g. U.S. Pat. No. 6,832,717), thepresence or absence of a macrodot was be used to represent a binary bit.However, the tag structure of the present invention encodes a data valueusing multi-pulse position modulation, which is described in more detailin Section 2.3.

The coding pattern 3 is represented on the surface in such a way as toallow it to be acquired by an optical imaging system, and in particularby an optical system with a narrowband response in the near-infrared.The pattern 3 is typically printed onto the surface using a narrowbandnear-infrared ink.

FIG. 3 shows the structure of a complete tag 4 with target elements 301shown. The tag 4 is square and contains sixteen target elements. Thosetarget elements 301 located at the edges and corners of the tag (twelvein total) are shared by adjacent tags and define the perimeter of thetag. The high number of target elements 301 advantageously facilitatesaccurate determination of a perspective distortion of the tag 4 when itis imaged by the sensing device 101. This improves the accuracy of tagsensing and, ultimately, position determination.

The tag 4 consists of a square array of nine symbol groups 303. Symbolgroups 303 are demarcated by the target elements 301 so that each symbolgroup is contained within a square defined by four target elements.Adjacent symbol groups 303 are contiguous and share targets.

Since the target elements 301 are all identical, they do not demarcateone tag from its adjacent tags. Viewed purely at the level of targetelements, only symbol groups 303, which define grid cells of a targetgrid, can be distinguished—the tags 4 themselves are indistinguishableby viewing only the target elements 301. Hence, tags 4 must be alignedwith the target grid as part of tag decoding.

The tag 4 is designed to allow all tag data to be recovered from animaging field of view substantially the size of the tag. This impliesthat any data unique to the tag 4 must appear four times within thetag—i.e. once in each quadrant or quarter; any data unique to a columnor row of tags must appear twice within the tag—i.e. once in eachhorizontal half or vertical half of the tag respectively; and any datacommon to a set of tags needs to appear once within the tag.

2.3 Symbol Groups

As shown in FIG. 4, each of the nine symbol groups 303 contains ten datasymbols 304, each data symbol being part of a codeword. In addition,each symbol group 303 comprises four registration symbols. These allowthe orientation and translation of the tag in the field of view to bedetermined, as well as the tag format to be determined Translationrefers to the translation of tag(s) relative to the symbol groups 303 inthe field of view. In other words, the registration symbols enablealignment of the ‘invisible’ tags with the target grid.

Each data symbol 304 is a multi-pulse position modulated (PPM) datasymbol. Each PPM data symbol 304 encodes a single 4-bit or 5-bitReed-Solomon symbol using 2 or 3 macrodots in any of 7 positions {p₀,p₁, p₂, p₃, p₄, p₅, p₆}, i.e. using 2-7 or 3-7 pulse-position modulation(PPM). 2-7 PPM is used if the tag format is 0; 3-7 PPM is used if thetag format is 1. 3-7 PPM has a range of 35 symbol values enabling 5-bitencoding with 3 unused symbol values, while 2-7 PPM has a range of 21values enabling 4-bit encoding with 5 unused symbol values.

FIG. 5 shows the layout for a first 7 PPM data symbol 304A, which issubstantially L-shaped.

FIG. 6 shows the layout for a second 7 PPM data symbol 304B, which issubstantially H-shaped. The differently shaped 7 PPM data symbols 304Aand 304B allow optimal tessellation of the data symbols when theyinterlock to form the coding pattern 3.

Table 1 defines the mapping from 2-7 PPM symbol values to Reed-Solomondata symbol values. Unused symbol values may be treated as erasures.

TABLE 1 2-7PPM symbol to 4-bit data symbol value mapping 2-7PPM symbol4-bit data symbol value (p₆-p₀) value (base 16) 0,000,011 unused0,000,101 0 0,000,110 unused 0,001,010 1 0,001,010 2 0,001,100 30,010,001 4 0,010,010 5 0,010,100 unused 0,011,000 6 0,100,001 70,100,010 8 0,100,100 9 0,101,000 a 0,110,000 b 1,000,001 c 1,000,010 d1,000,100 e 1,001,000 f 1,010,000 unused 1,100,000 unused

Unused PPM symbol values are chosen to avoid macrodot pairs along theconvex edge of the first L-shaped 7 PPM data symbol 304A shown in FIG.5, in order to avoid clustering or clumping of macrodots 302 betweenadjacent data symbols. Thus, the (p₀, p₁), (p₁, p₂), (p₂, p₄), (p₄, p₆)and (p₅, p₆) doublets are unused in 2-7 PPM encoding because thesedoublets are positioned along the convex edge of the first data symbol304A. With the tessellated tiling of data symbols 304 in the codingpattern, this non-arbitrary use of unused symbol values helps tominimize visibility of the coding pattern by maintaining a more evendistribution of macrodots.

Table 2 defines the mapping from 3-7 PPM symbol values to data symbolvalues. Unused symbol values may be treated as erasures

TABLE 2 3-7PPM symbol to 5-bit data symbol value mapping 3-7PPM symbol5-bit data symbol value (p₆-p₀) value (base 16) 0,000,111 0 0,001,011unused 0,001,101 1 0,001,110 2 0,010,011 3 0,010,101 4 0,010,110 unused0,011,001 5 0,011,010 6 0,011,100 7 0,100,011 8 0,100,101 9 0,100,110 a0,101,001 b 0,101,010 c 0,101,100 d 0,110,001 e 0,110,010 f 0,110,100 100,111,000 11 1,000,011 12 1,000,101 13 1,000,110 14 1,001,001 151,001,010 16 1,001,100 17 1,010,001 18 1,010,010 19 1,010,100 1a1,011,000 1b 1,100,001 1c 1,100,010 1d 1,100,100 1e 1,101,000 1f1,110,000 unused

Unused PPM symbol values are chosen to avoid macrodot triplets at thecorners of the first 7 PPM data symbol 304A shown in FIG. 4, in order toavoid clustering or clumping of macrodots 302 between adjacent datasymbols 304. Thus, the (p₀, p₁, p₃), (p₁, p₂, p₄) and (p₄, p₅, p₆)triplets are unused in 3-7 PPM encoding because these triplets arepositioned at the corners of the first data symbol 304A. With thetessellated tiling of data symbols 304 in the coding pattern, thisnon-arbitrary use of unused symbol values helps to minimize visibilityof the coding pattern by maintaining a more even distribution ofmacrodots.

2.4 Targets and Macrodots

The spacing of macrodots 302 in both dimensions, as shown in FIG. 7, isspecified by the parameter s. It has a nominal value of 127 μm, based on8 dots printed at a pitch of 1600 dots per inch.

Only the macrodots 302 are part of the representation of a data symbol304 in the coding pattern. The outline of a symbol 304 is shown in, forexample, FIGS. 3 and 4 merely to elucidate more clearly the structure ofa tag 4.

A macrodot 302 is nominally round with a nominal size of (⅝)s. However,it is allowed to vary in size by ±10% according to the capabilities ofthe device used to produce the pattern.

A target 301 is nominally circular with a nominal diameter of (12/8)s.However, it is allowed to vary in size by ±10% according to thecapabilities of the device used to produce the pattern.

Each tag 4 has a width of 30 s and a length of 30 s. However, it shouldbe noted from FIG. 3 that the tag 4 is configured so that some datasymbols extend beyond the perimeter edge of the tag 4 and interlock withcomplementary symbol groups from adjacent tags. This arrangementprovides a tessellated pattern of data symbols 304 within the codingpattern 3.

The macrodot spacing, and therefore the overall scale of the tagpattern, is allowed to vary by 120 μm and 127 μm according to thecapabilities of the device used to produce the pattern. Any deviationfrom the nominal scale is recorded in each tag (via a macrodot size IDfield) to allow accurate generation of position samples.

These tolerances are independent of one another. They may be refinedwith reference to particular printer characteristics.

2.5 Field of View

As mentioned above, the tag 4 is designed to allow all tag data to berecovered from an imaging field of view roughly the size of the tag. Anydata common to a set of contiguous tags only needs to appear once withineach tag, since fragments of the common data can be recovered fromadjacent tags. Any data common only to a column or row of tags mayappear twice within the tag—i.e. once in each horizontal half orvertical half of the tag respectively. Any data unique to the tag mustappear four times within the tag—i.e. once in each quadrant.

Although data which is common to a set of tags, in one or both spatialdimensions, may be decoded from fragments from adjacent tags,pulse-position modulated values are best decoded from spatially-coherentsamples (i.e. from a whole symbol as opposed to partial symbols atopposite sides of the field of view), since this allows raw samplevalues to be compared without first being normalised. This implies thatthe field of view must be large enough to contain two complete copies ofeach such pulse-position modulated value.

The tag is designed so that the maximum extent of a pulse-positionmodulated value is three macrodots (see FIG. 3). Thus, the minimumimaging field of view required to guarantee acquisition of an entire taghas a diameter of 46.7 s (i.e. (30+3)√2 s), allowing for arbitraryrotation and translation of the surface coding in the field of view.This field of view has a diameter of one tag plus one data symbol. Thisextra data symbol ensures that PPM data symbols can be decoded fromcontiguous macrodots.

Given a maximum macrodot spacing of 127 μm, the minimum required fieldof view has a diameter of 5.93 mm.

2.6 Encoded Codes and Codewords

In this following section (Section 2.6), each symbol in FIGS. 8 to 12 isshown with a unique label. The label consists of an alphabetic prefixwhich identifies which codeword the symbol is part of, and a numericsuffix which indicates the index of the symbol within the codeword. Theorientation of each symbol label indicates the orientation of thecorresponding symbol layout and is consistent with the symbolorientations shown in FIG. 4.

2.6.1 Designing an Optimized Registration Code

As discussed above, each tag 4 contains an array of square symbol groups303, with each symbol group being demarcated by a set of target elements301. In the present coding pattern 3 (known as “Cassia”), each tag 4contains an array of 3×3 symbol groups with each symbol group beingdemarcated by four target elements. However, it will be appreciated thatother configurations are of course possible e.g. 2×2, 4×4 etc.

The physical layout of the surface coding pattern consists of targetelements which define ‘grid cells’ of a target grid. Each grid cell ofthe target grid, when viewed purely at the level of target elements andmacrodot regions, may be described as a ‘physical symbol group’. Thephysical layout of the coding pattern is designed so that it belongs toa plane symmetry group that has rotational symmetry of the same order asthe number of targets delineating its physical symbol groups i.e.four-fold rotational symmetry. The physical layout also hastranslational symmetry with the physical symbol group as its unit cell.The present coding pattern has a physical layout which belongs tosymmetry group p4m. However, 4-fold rotational symmetry is not anessential requirement of coding patterns utilizing the presentregistration encoding. In alternative coding patterns, the physicallayout may be 2-fold, 3-fold or 6-fold rotationally symmetric. Examplesof such physical layouts are described in U.S. Pat. No. 7,111,791, thecontents of which is herein incorporated by reference.

However, the logical layout of a surface coding pattern (i.e. consistingof arrangements of data symbols and codewords—see FIG. 13) no longer hasthe same four-fold rotational symmetry as the physical layout. Rather,the logical layout is designed so that it belongs to a plane symmetrygroup that has translational symmetry with the tag, rather than thesymbol group, as its unit cell. The logical layout of the present codingpattern has the tag as its unit cell and a logical layout which belongsto symmetry group p1.

During decoding, the physical layout (i.e. the layout of 4-foldrotationally symmetric cells) is directly discernable from the physicalpattern of the targets 301. However, the rotation and translation of thelogical layout with respect to the physical layout must be determined bythe decoding process.

Assuming t targets per physical symbol group and c physical symbolgroups per tag, there are ct possible transformations (or registrations)between a physical layout and a logical layout. Thus, the present codingpattern has 4×3×3=36 possible registrations.

The Netpage surface coding includes a registration code that allows theregistration between the physical layout and the logical layout to bedetermined The physical layout of the registration code has the samesymmetry as the physical layout of the surface coding itself, while thelogical layout of the registration code has the same symmetry as thelogical layout of the surface coding. Crucially, however, theregistration code is designed so that a valid codeword of theregistration code can be read according to just the physical layout ofthe registration code, i.e. with an arbitrary registration, and thiscodeword will uniquely indicate the actual registration.

In the present coding pattern, the registration code contains oneregistration symbol per target within a symbol group, i.e. tregistration symbols per symbol group and therefore ct registrationsymbols per tag. Hence, the present coding pattern contains 36registration symbols. However, the number of registration symbols withineach symbol group may be any integer multiple n of the rotationalsymmetry. For example, in the 4-fold rotationally symmetric symbolgroups described herein, each symbol group may contain 4, 8, 12, 16 etcregistration symbols positioned in a 4-fold rotationally symmetricarrangement.

In addition to registration, it may be convenient to encode otherinformation in the registration code. For example, if a surface codingsupports multiple levels of redundancy and/or multiple modulationschemes for its data content, then the registration code can be used toencode the format of the tag data. It is also convenient to encode anactive area flag that indicates whether a particular tag is part of anactive area on the surface.

Since a surface coding is designed to be decodable from an imaging fieldof view (FOV) just large enough to contain a single tag, a decodergenerally utilizes fragments from adjacent tags. While the tag format iscommon to an entire surface region, the active area flag varies by tag.The active area flag is therefore more difficult to encode effectivelyin a registration code.

When the registration code has sufficient capacity it is convenient toencode registration, format and flag using a structured scheme that iseasy to decode. In the structured scheme, each symbol group contains aregistration code which independently identifies the registration,format and flag. However, when the registration code does not havesufficient capacity for a structured scheme it is convenient to use anunstructured scheme that can be optimized for the available capacity.The present invention employs an unstructured code, which has theadvantage of minimizing the data capacity of registration symbols.

The capacity of a registration code is a function of both its length andthe capacity of its symbols. Since its length is proportional to thenumber of registrations it must encode, the capacity of its symbolsdetermines its effective capacity.

Using d-in-p multi-pulse position modulation, the capacity C of a symbolis:C=p!/(p−d)!d!

Thus, registration symbols using 2-5 PPM have a capacity of 10 values,while reduced capacity registration symbols according to the presentinvention using 2-4 PPM have a capacity of 6 values (see Table 3 inSection 2.6.2).

An unstructured registration code is optimal in the sense that itserror-correcting capacity is maximal. Its error-correcting capacity ishigher than that of a structured registration code, but at the expenseof a more complex decoder. The decoder for a structured code typicallyconsists of a set of minimum-distance decoders operating on shortersub-codes within the code, while the decoder for an unstructured codetypically consists of a single minimum-distance decoder operating on theentire code.

A registration code that encodes just registration consists of ctcodewords, each of which corresponds to one of the possibletransformations of a single base codeword, as illustrated in FIG. 9. (InFIG. 9, the dotted arrow indicates distances between differentregistrations of the base codeword, with all distances being equal orexceeding d_(min)).

An optimal registration code is obtained by maximising the minimumdistance between its codewords.

No analytic method for obtaining an optimal registration code of acertain size and symbol capacity, nor even for determining its maximalminimum distance, is known to the present Applicant. The vector space ofpossible base codewords has a size of C^(ct). For typical code sizes andsymbol capacities this space is not amenable to exhaustive search.Useful registration codes can, however, be obtained by random search.

A registration code that encodes both registration and f possible tagformat values consists of ctf codewords, each of which corresponds toone of the ctf possible transformations of f base codewords, asillustrated in FIG. 10 (for f=2). (In FIG. 10, the dotted arrowsindicate distances between different registrations of base codewords,with solid arrows indicating the distance between base codewords and alldistances being equal or exceeding d_(min)).

An optimal format-encoding registration code is obtained by maximisingthe minimum distance between its codewords.

A registration code that encodes registration, f possible tag formatvalues, and g possible flag values consists of ctfg codewords, each ofwhich corresponds to one of the possible transformations of fg basecodewords, as illustrated in FIG. 11 (for f=2 and g=2). (In FIG. 11, thedotted arrows indicate distances between different registrations of basecodewords, with solid arrows indicating the distance between basecodwords and all distances being equal or exceeding d_(min)).

Because the flag value varies by tag, an optimal flag-encodingregistration code is no longer obtained simply by maximising the minimumdistance between its codewords. Instead, because a decoder needs to beable to decode the registration code from fragments from adjacent tags,the registration code needs to be designed so that any variation in flagvalue between adjacent tags does not excessively undermine registrationdecoding by introducing apparent errors.

The impact of flag value variation on the robustness of registrationdecoding can be reduced, at the expense of the robustness of flagdecoding, by selecting the base codewords so that the distance betweenbase codewords representing the same format value but different flagvalues is shorter than the minimum distance otherwise. This relativelyshorter distance between different flag base codewords is referred to asd_(flag), and is labelled as such in FIG. 11. In all other cases theminimum distance is relatively longer.

In practice, relatively less robust flag decoding is usually notproblematic. The flag is typically interpreted as a hint and cantherefore safely be interpreted as set if its value is ambiguous.

The registration code may additionally allow a reflection of the logicallayout relative to the physical layout to be determined The physicallayout has at least one 2D reflection axis so that the physical layoutis preserved in its mirror image, while the logical layout has noreflection axes. A modified registration code, which allows reflectionto be determined, is useful if the coding pattern is imaged inreflection, e.g. reflected in a mirror or through the back of atransparent substrate.

With at least one reflection axis per physical symbol group, t targetsper physical symbol group and c physical symbol groups per tag, therewill be 2ct possible registrations between a physical layout and alogical layout. Therefore, in this modified registration code, there are2×4×3×3=72 possible registrations.

Although the values of the 2-4 PPM registration symbols will bedifferent depending on whether they are read from the front or back ofthe substrate (i.e. reflected or not), this can be readily taken intoaccount when the code is optimized so as to maximally separate itscodewords. Accordingly, the differences in registration symbol values isnot an issue when reading the coding pattern from the front or back ofthe substrate.

2.6.2 Optimized Unstructured Registration Code

Each symbol group comprises four registration symbols, nominallydesignated R0, R1, R2, and R3. Each registration symbol 307 is encodedusing 2-4 PPM and so is limited to 6 symbol values. FIG. 12 shows thelayout of one 4 PPM registration symbol 307

As shown in FIG. 5 and FIG. 13, the registration symbols 307 each appearfour times within a symbol group.

Table 3 defines the mapping from 2-4 PPM symbol values to registrationsymbol values. Unused symbol values are treated as erasures.

TABLE 3 2-4PPM symbol to registration symbol mapping 2-4PPM symbolregistration value (p₃-p₀) symbol value 0011 0 0101 1 0110 2 1001 3 10104 1100 5

The registration code is defined by four base codewords, each with 36variants corresponding to the 36 possible registrations between a symbolgroup 303 and a tag 4. The registration code therefore includes 144codewords. (Each of the four base codewords would have 72 variants ifthe registration code allowed reflections to be determined, as describedin Section 2.6.1.)

A tag 4 contains a single base registration codeword consisting of 4registration symbols in each symbol group, i.e. 36 registration symbolsin all, as shown in FIG. 5 and FIG. 13.

The registration codeword of a tag encodes the orientation, translation,format and active area flag of the tag. Table 4 defines the four baseregistration codewords, each of which encodes one of the fourcombinations of tag format and active area flag.

TABLE 4 Base registration codewords active tag area codeword format flag4414, 0332, 2253, 1132, 1411, 3402, 5503, 2113, 2155 0 0 2414, 3332,4253, 4132, 4411, 5402, 4503, 1113, 3155 1 2321, 5555, 1101, 1212, 5500,4540, 5531, 4311, 4512 1 0 0321, 3555, 0101, 0212, 4500, 5540, 1531,5311, 5512 1

Each base codeword encodes zero rotation and translation, i.e. it isonly when a base codeword is read at a non-zero rotation and/ortranslation that the resulting variant of the base codeword encodes thecorresponding non-zero rotation and/or translation.

A base codeword is assigned to the registration symbols of a tagstarting with the left-most symbol of the codeword being assigned to R0of the top left symbol group of a tag, and then proceeding left-to-rightwithin the codeword and column-wise to the right and row-wise down inthe symbol group array of the tag.

The registration code is designed so that it has a minimum distance of26, with the exception of pairs of codewords that encode the sameregistration and tag format but different active area flags. Thedistance within these pairs (d_(flag)) is 9. This ensures that localvariations in the value of the active area flag do not destroy theerror-correcting capacity of the registration code.

A registration codeword is decoded using a minimum distance decoder. Inthe absence of active area flag variation the registration code has thecapacity to correct 12 errors. In the presence of active area flagvariation it has a worst-case capacity to correct 8 errors.

2.6.3 Coordinate Data

The tag contains an x-coordinate codeword and a y-coordinate codewordused to encode the x and y coordinates of the tag respectively. Thecodewords are of a shortened 2⁴-ary or 2⁵-ary (15, 3) Reed-Solomon code.The tag therefore encodes either two 12-bit or two 15-bit coordinates. A2⁴-ary code is used if the tag format is 0; a 2⁵-ary code is used if thetag format is 1.

The x coordinate codeword is constant within the column of tagscontaining the tag, while the y coordinate codeword is constant withinthe row of tags containing the tag.

The symbols of the coordinate codewords are evenly distributedthroughout the tag. This distribution ensures that an image of the tagpattern large enough to contain a complete tag is guaranteed to containat least 9 symbols of each coordinate codeword, irrespective of thealignment of the image with the tag pattern. Missing symbols must betreated as erasures. The (possibly punctured) instance of eithercoordinate codeword may consist of fragments from different tags. Notethat symbols of the x coordinate codeword placed in a particular columnof the tag are guaranteed to be acquired in their entirety from eitherthe tag or its immediate neighbour to the left or right, since the fieldof view is at least one symbol larger than the tag (as discussed inSection 2.5). The same applies to symbols of the y coordinate codewordplaced in a particular row of the tag.

The layout of the coordinate codewords is shown in FIG. 1. The codewordsshare the same layout, but are rotated 90 degrees relative to eachother.

2.6.4 Common Data

The tag contains three codewords A, B, C and D which encode informationcommon to a set of contiguous tags in a surface region. The codewordsare of a 2⁴-ary or shortened 2⁵-ary (15, 7) Reed-Solomon code. The tagtherefore encodes either 112 bits or 140 bits of information common to aset of contiguous tags. A 2⁴-ary code is used if the tag format is 0; a2⁵-ary code is used if the tag format is 1.

The common codewords are replicated throughout a tagged region. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of each commoncodeword, irrespective of the alignment of the image with the tagpattern. The instance of each common codeword may consist of fragmentsfrom different tags.

The layout of the common codewords is shown in FIG. 14. The codewordshave the same layout, rotated 90 degrees relative to each other.

2.6.5 Complete Tag

FIG. 15 shows the layout of the data of a complete tag, including datasymbols and registration symbols.

2.7 Error Detection and Correction

2.7.1 Reed-Solomon Encoding

All data is encoded using a Reed-Solomon code defined over GF(2^(m)),where the degree m=4 when the tag format is 0, and m=5 when the tagformat is 1.

The code has a natural length n of 2^(m)−1. The dimension k of the codeis chosen to balance the error correcting capacity and data capacity ofthe code, which are (n−k)/2 and k symbols respectively.

The code may be punctured, by removing high-order redundancy symbols, toobtain a code with reduced length and reduced error correcting capacity.The code may also be shortened, by replacing high-order data symbolswith zeros, to obtain a code with reduced length and reduced datacapacity. Both puncturing and shortening can be used to obtain a codewith particular parameters. Shortening is preferred, where possible,since this avoids the need for erasure decoding.

The code has the following primitive polynominals:p(x)=x ⁴ +x+1, when m=4p(x)=x ⁵ +x ²+1, when m=5

The code has the following generator polynominal:

${g(x)} = {\prod\limits_{i = 1}^{n - k}\left( {x + \alpha^{t}} \right)}$

For a detailed description of Reed-Solomon codes, refer to Wicker, S. B.and V. K. Bhargava, eds., Reed-Solomon Codes and Their Applications,IEEE Press, 1994.

2.7.2 Codeword Organization

As shown in FIG. 16, redundancy coordinates r_(i) and data coordinatesd_(i) of the code are indexed from left to right according to the powerof their corresponding polynomial terms. The symbols X_(i) of a completecodeword are indexed from right to left to match the bit order of thedata. The bit order within each symbol is the same as the overall bitorder.

2.7.3 Code Instances

Table 5 defines the parameters of the different codes used in the tag.

TABLE 5 Codeword instances error- correcting data codeword codewordlength dimension capacity degree capacity^(a) name description (n) (k)(symbols) (m) (bits) X, Y coordinate 9-15^(a) 3 3-6 4 12 codewords 5 15(see Section 2.6.3) A, B, C, D common 15 7 4 4 28 codewords 5 35 (seeSection 2.6.4) ^(a)Effective codeword length depends on codewordvisibility within field of view.2.7.4 Cyclic Redundancy Check

The region ID encoded by the common codewords is protected by a 16-bitcyclic redundancy check (CRC). This provides an added layer of errordetection after Reed-Solomon error correction, in case a codewordcontaining a part of the region ID is mis-corrected.

The CRC has the following generator polynomial:g(x)=x ¹⁶ +x ¹² +x ⁵+1

The CRC is initialised to 0xFFFF. The most significant bit of the regionID is treated as the most significant coefficient of the datapolynomial.

2.8 Tag Coordinate Space

The tag coordinate space has two orthogonal axes labelled x and yrespectively. When the positive x axis points to the right then thepositive y axis points down.

The surface coding does not specify the location of the tag coordinatespace origin on a particular tagged surface, nor the orientation of thetag coordinate space with respect to the surface. This information isapplication-specific. For example, if the tagged surface is a sheet ofpaper, then the application which prints the tags onto the paper mayrecord the actual offset and orientation, and these can be used tonormalise any digital ink subsequently captured in conjunction with thesurface.

The position encoded in a tag is defined in units of tags and is definedto be the centre of the top left target. The origin of a particular tagpattern is therefore the centre of the top left target of the tag thatencodes coordinate pair (0, 0).

The surface coding is optionally displaced from its nominal positionrelative to the surface by an amount derived from the region ID. Thisensures that the utilisation of a pagewidth digital printhead used toprint the surface coding is uniform. The displacement of the surfacecoding is negative, hence the displacement of the region described bythe surface coding is positive relative to the surface coding. Themagnitude of the displacement is the region ID modulo the width of thetag in 1600 dpi dots (i.e. 240). To accommodate non-1600 dpi printersthe actual magnitude of the displacement may vary from its nominal valueby up to half the dot pitch of the printer.

2.9 Tag Information Content

2.9.1 Field Definitions

Table 6 defines the information fields embedded in the surface coding.

TABLE 6 Field Definitions width field (bits) description unique to tagactive area flag 1 A flag indicating whether the area^(a) immediatelysurrounding a tag intersects an active area. x coordinate 12 or 15 Theunsigned x coordinate of the tag^(b). y coordinate 12 or 15 The unsignedy coordinate of the tag^(b). common to tagged region tag format 1 Theformat of the tag: 0: 2-7PPM, m = 4 1: 3-7PPM, m = 5 encoding format 2The format of the encoding. 0: the present encoding. Other values arereserved region flags 10  Flags controlling the interpretation of regiondata (see Table 7). macrodot size ID 4 The ID of the macrodot size.region ID  80 or 108 The ID of the region containing the tags. CRC(Cyclic 16  A CRC of the region ID Redundancy (see Section 2.7.4).Check) ^(a)the diameter of the area, centered on the tag, is nominally2.5 times the diagonal size of the tag; this is to accommodate theworst-case distance between the nib position and the imaged tag^(b)allows a coordinate value ranges of 15 m and 118 km respectively forthe minimum tag size of 3.6 mm (based on the minimum macrodot size of120 microns and 30 macrodots per tag)

An active area is an area within which any captured input should beimmediately forwarded to the corresponding Netpage server 10 forinterpretation. This also allows the Netpage server 10 to signal to theuser that the input has had an immediate effect. Since the server hasaccess to precise region definitions, any active area indication in thesurface coding can be imprecise so long as it is inclusive.

TABLE 7 Region flags bit meaning 0 Region is interactive, i.e. x andy-coordinates are present. 1 Region is active, i.e. the entire region isan active area. Otherwise active areas are identified by individualtags' active area flags. 2 Region ID is serialized^(a). 3 Region IDcontains a digital signature^(b) 4 Region ID is an EPC 5 Region isdisplaced according to region ID (see Section 2.8) other Reserved forfuture use ^(a)If not set for an EPC this means that the serial numberis replaced by a layout number, to allow the package design associatedwith a product to vary over time (see US 2007/0108285, the contents ofwhich is herein incorporated by reference). ^(b)Hence the region IDshould not be transmitted in the clear during resolution.2.9.2 Mapping of Fields to Codewords

Table 8 and Table 9 define how the information fields map to codewords.

TABLE 8 Mapping of fields to coordinate codewords X and Y codewordcodeword field tag format field width field bits bits X x coordinate 012 all all 1 15 Y y coordinate 0 12 all all 1 15

TABLE 9 Mapping of fields to common codewords A, B, C and D tag fieldcodeword codeword field format field width bits bits A CRC any 16 all15:0  region ID 0 12 11:0  27:16 1 19 18:0  34:16 B encoding format any2 all 1:0 region flags any 10 all 11:2  macrodot size ID any 4 all 15:12region ID 0 12 23:12 27:16 1 19 37:19 34:16 C region ID 0 28 51:24 all 135 72:38 all D region ID 0 28 79:52 all 1 35 107:73  all

When the region flags indicate that a particular codeword is absent thenthe codeword is not coded in the tag pattern, i.e. there are nomacrodots representing the codeword. This applies to the X and Y i.e.the X and Y codewords are present if the <region is interactive> flag inthe region flags is set.

2.10 Tag Imaging and Decoding

As explained above, the minimum imaging field of view required toguarantee acquisition of data from an entire tag has a diameter of 47.6s (i.e. (30+3)√2 s), allowing for arbitrary rotation and translation ofthe surface coding in the field of view. Notably, the imaging field ofview does not have to be large enough to guarantee capture of an entiretag—the arrangement of the data symbols within each tag ensures that aany square portion of length (l+3 s) captures the requisite informationin full, irrespective of whether a whole tag is actually visible in thefield-of-view. As used herein, l is defined as the length of a tag.

The extra three macrodot units ensure that pulse-position modulatedvalues can be decoded from spatially coherent samples. Furthermore, theextra three macrodot units ensure that all requisite data symbols can beread with each interaction. These include the coordinate symbols from acentral column or row of a tag (see Section 2.6.3) having a width of 3s.

In the present context, a “tag diameter” is given to mean the length ofa tag diagonal.

FIG. 17 shows a tag image processing and decoding process flow up to thestage of sampling and decoding the data codewords. Firstly, a raw image802 of the tag pattern is acquired (at 800), for example via an imagesensor such as a CCD image sensor, CMOS image sensor, or a scanninglaser and photodiode image sensor. The raw image 802 is then typicallyenhanced (at 804) to produce an enhanced image 806 with improvedcontrast and more uniform pixel intensities. Image enhancement mayinclude global or local range expansion, equalisation, and the like. Theenhanced image 806 is then typically filtered (at 808) to produce afiltered image 810. Image filtering may consist of low-pass filtering,with the low-pass filter kernel size tuned to obscure macrodots 302 butto preserve targets 301. The filtering step 808 may include additionalfiltering (such as edge detection) to enhance target features 301.Encoding of data codewords 304 using pulse position modulation (PPM)provides a more uniform coding pattern 3 than simple binary dot encoding(as described in, for example, U.S. Pat. No. 6,832,717). Advantageously,this helps separate targets 301 from data areas, thereby allowing moreeffective low-pass filtering of the PPM-encoded data compared tobinary-coded data.

Following low-pass filtering, the filtered image 810 is then processed(at 812) to locate the targets 301. This may consist of a search fortarget features whose spatial inter-relationship is consistent with theknown geometry of the tag pattern. Candidate targets may be identifieddirectly from maxima in the filtered image 810, or may be the subject offurther characterization and matching, such as via their (binary orgrayscale) shape moments (typically computed from pixels in the enhancedimage 806 based on local maxima in the filtered image 810), as describedin U.S. Pat. No. 7,055,739, the contents of which is herein incorporatedby reference.

The identified targets 301 are then assigned (at 816) to a target grid818. At this stage, individual tags 4 will not be identifiable in thetarget grid 818, because the targets 301 do not demarcate one tag fromanother.

To allow macrodot values to be sampled accurately, the perspectivetransform of the captured image must be inferred. Four of the targets301 are taken to be the perspective-distorted corners of a square ofknown size in tag space, and the eight-degree-of-freedom perspectivetransform 822 is inferred (at 820), based on solving the well-understoodequations relating the four tag-space and image-space point pairs.Calculation of the 2D perspective transform is described in detail in,for example, Applicant's U.S. Pat. No. 6,832,717, the contents of whichis herein incorporated by reference.

The inferred tag-space to image-space perspective transform 822 is usedto project each known macrodot position in tag space into image space.Since all bits in the tags are represented by PPM-encoding, the presenceor absence of each macrodot 302 can be determined using a localintensity reference, rather than a separate intensity reference. Thus,PPM-encoding provides improved data sampling compared with pure binaryencoding.

At the next stage, at least 36 registration symbols visible within thefield of view are sampled and decoded (at 824). Decoding of the 36registration symbols enables to the pen to construct a sampledregistration codeword consisting of 36 registration symbol valuesarranged in a sequence determined by reading each registration symbolwithin the field of view in a predetermined order.

The sampled registration codeword is identified as a variant of one ofthe four base registration codewords defined in Table 4. As discussedabove in Section 2.6.2, identification of one of the four baseregistration codewords enables the tag format code (0 or 1) and flagcode (0 or 1) to be determined (at 830). Moreover, the particularvariant of the base registration codeword corresponding to the sampledregistration codeword enables the registration (i.e. the verticaltranslation, horizontal translation and orientation) to be determined(at 830).

The horizontal and vertical translations (identified from the sampledregistration codeword) are used to determine the translation of tags(s)(or portions thereof) in the field of view relative to the target grid818. This enables alignment of the tags 4 with the target grid 818,thereby allowing individual tag(s), or portions thereof, to bedistinguished in the coding pattern 3 in the field of view. Usually, awhole tag will not be visible in the field of view although it is, ofcourse, possible for a whole tag to be visible when the field of view isaligned with the coding pattern at zero horizontal and verticaltranslations.

The orientation (identified from the sampled registration codeword) isused to determine the orientation of the data symbols relative to thetarget grid 818.

The flag code is used to identify the presence of absence of an activearea flag in the tag (or part thereof) within the field of view.

The tag format code is used to determine (at 825) the data symbolmodulation. The tag format codes of 0 and 1 identify 2-7 PPM and 3-7 PPMencoding of data symbols, respectively (at 826). The type of PPMencoding needs to be known before decoding of the data symbols (at 836).

Once initial imaging and decoding has yielded the 2D perspectivetransform, the orientation, the translation of tag(s) relative to thetarget grid and the format of the data symbols, the data codewords 304can then be sampled and decoded (at 836) to yield the requisite decodedcodewords 838.

Decoding of the data codewords 304 typically proceeds as follows:

-   -   sample and decode Reed-Solomon codeword containing common data        (A, B, C and D)    -   verify CRC of common data    -   on decode error flag bad region ID sample    -   determine region ID    -   sample and decode x and y coordinate Reed-Solomon codewords (X        and Y)    -   determine tag x-y location from codewords    -   determine nib x-y location from tag x-y location and perspective        transform taking into account macrodot size (from macrodot size        ID)    -   determine active area status of nib location with reference to        active area flag identified from registration codeword    -   encode region ID, nib x-y location, and nib active area status        in digital ink (“interaction data”)

In practice, when decoding a sequence of images of a tag pattern, it isuseful to exploit inter-frame coherence to obtain greater effectiveredundancy.

Region ID decoding need not occur at the same rate as position decoding.

The skilled person will appreciate that the decoding sequence describedabove represents one embodiment of the present invention. It will, ofcourse, be appreciated that the digital ink (“interaction data”) sentfrom the pen 101 to the netpage system in the form of digital ink mayinclude other data e.g. a digital signature, pen mode (see US2007/125860), orientation data, pen ID, nib ID etc.

An example of interpreting digital ink, received by the netpage systemfrom the netpage pen 101, is discussed briefly above. A more detaileddiscussion of how the netpage system may interpret interaction data canbe found in the Applicant's previously-filed applications (see, forexample, US 2007/130117 and US 2007/108285, the contents of which areherein incorporated by reference).

3. Netpage Pen

3.1 Functional Overview

The active sensing device of the netpage system may take the form of aclicker (for clicking on a specific position on a surface), a pointerhaving a stylus (for pointing or gesturing on a surface using pointerstrokes), or a pen having a marking nib (for marking a surface with inkwhen pointing, gesturing or writing on the surface). For a descriptionof various netpage sensing devices, reference is made to U.S. Pat. No.7,105,753; U.S. Pat. No. 7,015,901; U.S. Pat. No. 7,091,960; and USPublication No. 2006/0028459, the contents of each of which are hereinincorporated by reference.

It will be appreciated that the present invention may utilize anysuitable optical reader. However, the Netpage pen 400 will be describedherein as one such example.

The Netpage pen 400 is a motion-sensing writing instrument which worksin conjunction with a tagged Netpage surface (see Section 2). The penincorporates a conventional ballpoint pen cartridge for marking thesurface, an image sensor and processor for simultaneously capturing theabsolute path of the pen on the surface and identifying the surface, aforce sensor for simultaneously measuring the force exerted on the nib,and a real-time clock for simultaneously measuring the passage of time.

While in contact with a tagged surface, as indicated by the forcesensor, the pen continuously images the surface region adjacent to thenib, and decodes the nearest tag in its field of view to determine boththe identity of the surface, its own instantaneous position on thesurface and the pose of the pen. The pen thus generates a stream oftimestamped position samples relative to a particular surface, andtransmits this stream to the Netpage server 10. The sample streamdescribes a series of strokes, and is conventionally referred to asdigital ink (DInk). Each stroke is delimited by a pen down and a pen upevent, as detected by the force sensor. More generally, any dataresulting from an interaction with a Netpage, and transmitted to theNetpage server 10, is referred to herein as “interaction data”.

The pen samples its position at a sufficiently high rate (nominally 100Hz) to allow a Netpage server to accurately reproduce hand-drawnstrokes, recognise handwritten text, and verify hand-written signatures.

The Netpage pen also supports hover mode in interactive applications. Inhover mode the pen is not in contact with the paper and may be somesmall distance above the surface of the paper (or other substrate). Thisallows the position of the pen, including its height and pose to bereported. In the case of an interactive application the hover modebehaviour can be used to move a cursor without marking the paper, or thedistance of the nib from the coded surface could be used for toolbehaviour control, for example an air brush function.

The pen includes a Bluetooth radio transceiver for transmitting digitalink via a relay device to a Netpage server. When operating offline froma Netpage server the pen buffers captured digital ink in non-volatilememory. When operating online to a Netpage server the pen transmitsdigital ink in real time.

The pen is supplied with a docking cradle or “pod”. The pod contains aBluetooth to USB relay. The pod is connected via a USB cable to acomputer which provides communications support for local applicationsand access to Netpage services.

The pen is powered by a rechargeable battery. The battery is notaccessible to or replaceable by the user. Power to charge the pen can betaken from the USB connection or from an external power adapter throughthe pod. The pen also has a power and USB-compatible data socket toallow it to be externally connected and powered while in use.

The pen cap serves the dual purpose of protecting the nib and theimaging optics when the cap is fitted and signalling the pen to leave apower-preserving state when uncapped.

3.2 Ergonomics and Layout

FIG. 18 shows a rounded triangular profile gives the pen 400 anergonomically comfortable shape to grip and use the pen in the correctfunctional orientation. It is also a practical shape for accommodatingthe internal components. A normal pen-like grip naturally conforms to atriangular shape between thumb 402, index finger 404 and middle finger406.

As shown in FIG. 19, a typical user writes with the pen 400 at a nominalpitch of about 30 degrees from the normal toward the hand 408 when held(positive angle) but seldom operates a pen at more than about 10 degreesof negative pitch (away from the hand). The range of pitch angles overwhich the pen 400 is able to image the pattern on the paper has beenoptimised for this asymmetric usage. The shape of the pen 400 helps toorient the pen correctly in the user's hand 408 and to discourage theuser from using the pen “upside-down”. The pen functions “upside-down”but the allowable tilt angle range is reduced.

The cap 410 is designed to fit over the top end of the pen 400, allowingit to be securely stowed while the pen is in use. Multi colour LEDsilluminate a status window 412 in the top edge (as in the apex of therounded triangular cross section) of the pen 400 near its top end. Thestatus window 412 remains un-obscured when the cap is stowed. Avibration motor is also included in the pen as a haptic feedback system(described in detail below).

As shown in FIG. 20, the grip portion of the pen has a hollow chassismolding 416 enclosed by a base molding 528 to house the othercomponents. The ink cartridge 414 for the ball point nib (not shown)fits naturally into the apex 420 of the triangular cross section,placing it consistently with the user's grip. This in turn providesspace for the main PCB 422 in the centre of the pen and for the battery424 in the base of the pen. By referring to FIG. 21A, it can be seenthat this also naturally places the tag-sensing optics 426 unobtrusivelybelow the nib 418 (with respect to nominal pitch). The nib molding 428of the pen 400 is swept back below the ink cartridge 414 to preventcontact between the nib molding 428 and the paper surface when the penis operated at maximum pitch.

As best shown in FIG. 21B, the imaging field of view 430 emerges througha centrally positioned IR filter/window 432 below the nib 418, and twonear-infrared illumination LEDs 434, 436 emerge from the two bottomcorners of the nib molding 428. Each LED 434, 436 has a correspondingillumination field 438, 440.

As the pen is hand-held, it may be held at an angle that causesreflections from one of the LED's that are detrimental to the imagesensor. By providing more than one LED, the LED causing the offendingreflections can be extinguished.

Specific details of the pen mechanical design can be found in USPublication No. 2006/0028459, the contents of which are hereinincorporated by reference.

3.3 Pen Feedback Indications

FIG. 22 is a longitudinal cross section through the centre-line if thepen 400 (with the cap 410 stowed on the end of the pen). The penincorporates red and green LEDs 444 to indicate several states, usingcolours and intensity modulation. A light pipe 448 on the LEDs 444transmit the signal to the status indicator window 412 in the tubemolding 416. These signal status information to the user includingpower-on, battery level, untransmitted digital ink, network connectionon-line, fault or error with an action, detection of an “active area”flag, detection of an “embedded data” flag, further data sampling torequired to acquire embedded data, acquisition of embedded datacompleted etc.

A vibration motor 446 is used to haptically convey information to theuser for important verification functions during transactions. Thissystem is used for important interactive indications that might bemissed due to inattention to the LED indicators 444 or high levels ofambient light. The haptic system indicates to the user when:

-   -   The pen wakes from standby mode    -   There is an error with an action    -   To acknowledge a transaction        3.4 Pen Optics

The pen incorporates a fixed-focus narrowband infrared imaging system.It utilizes a camera with a short exposure time, small aperture, andbright synchronised illumination to capture sharp images unaffected bydefocus blur or motion blur.

TABLE 10 Optical Specifications Magnification ^(~)0.225 Focal length of6.0 mm lens Viewing distance 30.5 mm Total track length 41.0 mm Aperturediameter 0.8 mm Depth of field .^(~)/6.5 mm Exposure time 200 usWavelength 810 nm Image sensor size 140 × 140 pixels Pixel size 10 umPitch range ^(~)15

 45 deg Roll range ^(~)30

 30 deg Yaw range 0 to 360 deg Minimum sampling 2.25 pixels per ratemacrodot Maximum pen 0.5 m/s velocity ¹Allowing 70 micron blur radius²Illumination and filter ³Pitch, roll and yaw are relative to the axisof the pen

Cross sections showing the pen optics are provided in FIGS. 23A and 23B.An image of the Netpage tags printed on a surface 548 adjacent to thenib 418 is focused by a lens 488 onto the active region of an imagesensor 490. A small aperture 494 ensures the available depth of fieldaccommodates the required pitch and roll ranges of the pen 400.

First and second LEDs 434 and 436 brightly illuminate the surface 549within the field of view 430. The spectral emission peak of the LEDs ismatched to the spectral absorption peak of the infrared ink used toprint Netpage tags to maximise contrast in captured images of tags. Thebrightness of the LEDs is matched to the small aperture size and shortexposure time required to minimise defocus and motion blur.

A longpass IR filter 432 suppresses the response of the image sensor 490to any coloured graphics or text spatially coincident with imaged tagsand any ambient illumination below the cut-off wavelength of the filter432. The transmission of the filter 432 is matched to the spectralabsorption peak of the infrared ink to maximise contrast in capturedimages of tags. The filter also acts as a robust physical window,preventing contaminants from entering the optical assembly 470.

3.5 Pen Imaging System

A ray trace of the optic path is shown in FIG. 24. The image sensor 490is a CMOS image sensor with an active region of 140 pixels squared. Eachpixel is 10 μm squared, with a fill factor of 93%. Turning to FIG. 25,the lens 488 is shown in detail. The dimensions are:

-   -   D=3 mm    -   R1=3.593 mm    -   R2=15.0 mm    -   X=0.8246 mm    -   Y=10 mm    -   Z=0.25 mm

This gives a focal length of 6.15 mm and transfers the image from theobject plane (tagged surface 548) to the image plane (image sensor 490)with the correct sampling frequency to successfully decode all imagesover the specified pitch, roll and yaw ranges. The lens 488 is biconvex,with the most curved surface facing the image sensor. The minimumimaging field of view 430 required to guarantee acquisition ofsufficient tag data with each interaction is dependent on the specificcoding pattern. The required field of view for the coding pattern of thepresent invention is described in Section 2.10.

The required paraxial magnification of the optical system is defined bythe minimum spatial sampling frequency of 2.25 pixels per macrodot forthe fully specified tilt range of the pen 400, for the image sensor 490of 10 μm pixels. Typically, the imaging system employs a paraxialmagnification of 0.225, the ratio of the diameter of the inverted imageat the image sensor to the diameter of the field of view at the objectplane, on an image sensor 490 of minimum 128×128 pixels. The imagesensor 490 however is 140×140 pixels, in order to accommodatemanufacturing tolerances. This allows up to +/−120 μm (12 pixels in eachdirection in the plane of the image sensor) of misalignment between theoptical axis and the image sensor axis without losing any of theinformation in the field of view.

The lens 488 is made from Poly-methyl-methacrylate (PMMA), typicallyused for injection moulded optical components. PMMA is scratchresistant, and has a refractive index of 1.49, with 90% transmission at810 nm. The lens is biconvex to assist moulding precision and features amounting surface to precisely mate the lens with the optical barrelmolding 492.

A 0.8 mm diameter aperture 494 is used to provide the depth of fieldrequirements of the design.

The specified tilt range of the pen is 15.0 to 45.0 degree pitch, with aroll range of 30.0 to 30.0 degrees. Tilting the pen through itsspecified range moves the tilted object plane up to 6.3 mm away from thefocal plane. The specified aperture thus provides a corresponding depthof field of /6.5 mm, with an acceptable blur radius at the image sensorof 16 μm.

Due to the geometry of the pen design, the pen operates correctly over apitch range of 33.0 to 45.0 degrees.

Referring to FIG. 26, the optical axis 550 is pitched 0.8 degrees awayfrom the nib axis 552. The optical axis and the nib axis converge towardthe paper surface 548. With the nib axis 552 perpendicular to the paper,the distance A between the edge of the field of view 430 closest to thenib axis and the nib axis itself is 1.2 mm.

The longpass IR filter 432 is made of CR-39, a lightweight thermosetplastic heavily resistant to abrasion and chemicals such as acetone.Because of these properties, the filter also serves as a window. Thefilter is 1.5 mm thick, with a refractive index of 1.50. Each filter maybe easily cut from a large sheet using a CO₂ laser cutter.

3.6 Electronics Design

TABLE 11 Electrical Specifications Processor ARM7 (Atmel AT91FR40162)running at 80 MHz with 256 kB SRAM and 2 MB flash memory Digital inkstorage 5 hours of writing capacity Bluetooth 1.2 Compliance USBCompliance 1.1 Battery standby 12 hours (cap off), >4 weeks (cap on)time Battery writing 4 hours of cursive writing (81% pen down, timeassuming easy offload of digital ink) Battery charging 2 hours timeBattery Life Typically 300 charging cycles or 2 years (whichever occursfirst) to 80% of initial capacity. Battery ~340 mAh at 3.7 V,Lithium-ion Polymer Capacity/Type (LiPo)

FIG. 27 is a block diagram of the pen electronics. The electronicsdesign for the pen is based around five main sections. These are:

-   -   the main ARM7 microprocessor 574,    -   the image sensor and image processor 576,    -   the Bluetooth communications module 578,    -   the power management unit IC (PMU) 580 and    -   the force sensor microprocessor 582.        3.6.1 Microprocessor

The pen uses an Atmel AT91FR40162 microprocessor (see Atmel, AT91 ARMThumb Microcontrollers—AT91FR40162 Preliminary,http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at80 MHz. The AT91FR40162 incorporates an ARM7 microprocessor, 256 kBytesof on-chip single wait state SRAM and 2 MBytes of external flash memoryin a stack chip package.

This microprocessor 574 forms the core of the pen 400. Its dutiesinclude:

-   -   setting up the Jupiter image sensor 584,    -   decoding images of Netpage coding pattern (see Section 2.10),        with assistance from the image processing features of the image        sensor 584, for inclusion in the digital ink stream along with        force sensor data received from the force sensor microprocessor        582,    -   setting up the power management IC (PMU) 580,    -   compressing and sending digital ink via the Bluetooth        communications module 578, and    -   programming the force sensor microprocessor 582.

The ARM7 microprocessor 574 runs from an 80 MHz oscillator. Itcommunicates with the Jupiter image sensor 576 using a UniversalSynchronous Receiver Transmitter (USRT) 586 with a 40 MHz clock. TheARM7 574 communicates with the Bluetooth module 578 using a UniversalAsynchronous Receiver Transmitter (UART) 588 running at 115.2 kbaud.Communications to the PMU 580 and the Force Sensor microProcessor (FSP)582 are performed using a Low Speed Serial bus (LSS) 590. The LSS isimplemented in software and uses two of the microprocessor's generalpurpose IOs.

The ARM7 microprocessor 574 is programmed via its JTAG port.

3.6.2 Image Sensor

The ‘Jupiter’ Image Sensor 584 (see US Publication No. 2005/0024510, thecontents of which are incorporated herein by reference) contains amonochrome sensor array, an analogue to digital converter (ADC), a framestore buffer, a simple image processor and a phase lock loop (PLL). Inthe pen, Jupiter uses the USRT's clock line and its internal PLL togenerate all its clocking requirements. Images captured by the sensorarray are stored in the frame store buffer. These images are decoded bythe ARM7 microprocessor 574 with help from the ‘Callisto’ imageprocessor contained in Jupiter. The Callisto image processor performs,inter alia, low-pass filtering of captured images (see Section 2.10 andUS Publication No. 2005/0024510) before macrodot sampling and decodingby the microprocessor 574.

Jupiter controls the strobing of two infrared LEDs 434 and 436 at thesame time as its image array is exposed. One or other of these twoinfrared LEDs may be turned off while the image array is exposed toprevent specular reflection off the paper that can occur at certainangles.

3.6.3 Bluetooth Communications Module

The pen uses a CSR BlueCore4-External device (see CSR,BlueCore4-External Data Sheet rev c, 6-Sep.-2004) as the Bluetoothcontroller 578. It requires an external 8 Mbit flash memory device 594to hold its program code. The BlueCore4 meets the Bluetooth v1.2specification and is compliant to v0.9 of the Enhanced Data Rate (EDR)specification which allows communication at up to 3 Mbps.

A 2.45 GHz chip antenna 486 is used on the pen for the Bluetoothcommunications.

The BlueCore4 is capable of forming a UART to USB bridge. This is usedto allow USB communications via data/power socket 458 at the top of thepen 456.

Alternatives to Bluetooth include wireless LAN and PAN standards such asIEEE 802.11 (Wi-Fi) (see IEEE, 802.11 Wireless Local Area Networks,http://grouper.ieee.org/groups/802/11/index.html), IEEE 802.15 (seeIEEE, 802.15 Working Group for WPAN,http://grouperieee.org/groups/802/15/index.html), ZigBee (see ZigBeeAlliance, http://www.zigbee.org), and WirelessUSB Cypress (seeWirelessUSB LR 2.4-GHz DSSS Radio SoC,http://www.cypress.com/cfuploads/img/products/cywusb6935.pdf), as wellas mobile standards such as GSM (see GSM Association,http://www.gsmworld.com/index.shtml), GPRS/EDGE, GPRS Platform,http://www.gsmworld.com/technology/gprs/index.shtml), CDMA (see CDMADevelopment Group, http://www.cdg.org/, and Qualcomm,http://www.qualcomm.com), and UMTS (see 3rd Generation PartnershipProject (3GPP), http://www.3gpp.org).

3.6.4 Power Management Chip

The pen uses an Austria Microsystems AS3603 PMU 580 (see AustriaMicrosystems, AS3603 Multi-Standard Power Management Unit Data Sheetv2.0). The PMU is used for battery management, voltage generation, powerup reset generation and driving indicator LEDs and the vibrator motor.

The PMU 580 communicates with the ARM7 microprocessor 574 via the LSSbus 590.

3.6.5 Force Sensor Subsystem

The force sensor subsystem comprises a custom Hokuriku force sensor 500(based on Hokuriku, HFD-500 Force Sensor,http://www.hdk.co.jp/pdf/eng/e1381AA.pdf), an amplifier and low passfilter 600 implemented using op-amps and a force sensor microprocessor582.

The pen uses a Silicon Laboratories C8051F330 as the force sensormicroprocessor 582 (see Silicon Laboratories, C8051F330/1 MCU DataSheet, rev 1.1). The C8051F330 is an 8051 microprocessor with on chipflash memory, 10 bit ADC and 10 bit DAC. It contains an internal 24.5MHz oscillator and also uses an external 32.768 kHz tuning fork.

The Hokuriku force sensor 500 is a silicon piezoresistive bridge sensor.An op-amp stage 600 amplifies and low pass (anti-alias) filters theforce sensor output. This signal is then sampled by the force sensormicroprocessor 582 at 5 kHz.

Alternatives to piezoresistive force sensing include capacitive andinductive force sensing (see Wacom, “Variable capacity condenser andpointer”, US Patent Application 20010038384, filed 8 Nov. 2001, andWacom, Technology, http://www.wacom-components.com/english/tech.asp).

The force sensor microprocessor 582 performs further (digital) filteringof the force signal and produces the force sensor values for the digitalink stream. A frame sync signal from the Jupiter image sensor 576 isused to trigger the generation of each force sample for the digital inkstream. The temperature is measured via the force sensormicroprocessor's 582 on chip temperature sensor and this is used tocompensate for the temperature dependence of the force sensor andamplifier. The offset of the force signal is dynamically controlled byinput of the microprocessor's DAC output into the amplifier stage 600.

The force sensor microprocessor 582 communicates with the ARM7microprocessor 574 via the LSS bus 590. There are two separate interruptlines from the force sensor microprocessor 582 to the ARM7microprocessor 574. One is used to indicate that a force sensor sampleis ready for reading and the other to indicate that a pen down/up eventhas occurred.

The force sensor microprocessor flash memory is programmed in-circuit bythe ARM7 microprocessor 574.

The force sensor microprocessor 582 also provides the real time clockfunctionality for the pen 400. The RTC function is performed in one ofthe microprocessor's counter timers and runs from the external 32.768kHz tuning fork. As a result, the force sensor microprocessor needs toremain on when the cap 472 is on and the ARM7 574 is powered down. Hencethe force sensor microprocessor 582 uses a low power LDO separate fromthe PMU 580 as its power source. The real time clock functionalityincludes an interrupt which can be programmed to power up the ARM7 574.

The cap switch 602 is monitored by the force sensor microprocessor 582.When the cap assembly 472 is taken off (or there is a real time clockinterrupt), the force sensor microprocessor 582 starts up the ARM7 572by initiating a power on and reset cycle in the PMU 580.

3.7 Pen Software

The Netpage pen software comprises that software running onmicroprocessors in the Netpage pen 400 and Netpage pod.

The pen contains a number of microprocessors, as detailed in Section3.6. The Netpage pen software includes software running on the AtmelARM7 CPU 574 (hereafter CPU), the Force Sensor microprocessor 582, andalso software running in the VM on the CSR BlueCore Bluetooth module 578(hereafter pen BlueCore). Each of these processors has an associatedflash memory which stores the processor specific software, together withsettings and other persistent data. The pen BlueCore 578 also runsfirmware supplied by the module manufacturer, and this firmware is notconsidered a part of the Netpage pen software.

The pod contains a CSR BlueCore Bluetooth module (hereafter podBlueCore). The Netpage pen software also includes software running inthe VM on the pod BlueCore.

As the Netpage pen 400 traverses a Netpage tagged surface 548, a streamof correlated position and force samples are produced. This stream isreferred to as DInk. Note that DInk may include samples with zero force(so called “Hover DInk”) produced when the Netpage pen is in proximityto, but not marking, a Netpage tagged surface.

The CPU component of the Netpage pen software is responsible for DInkcapture, tag image processing and decoding (in conjunction with theJupiter image sensor 576), storage and offload management, hostcommunications, user feedback and software upgrade. It includes anoperating system (RTOS) and relevant hardware drivers. In addition, itprovides a manufacturing and maintenance mode for calibration,configuration or detailed (non-field) fault diagnosis. The Force Sensormicroprocessor 582 component of the Netpage pen software is responsiblefor filtering and preparing force samples for the main CPU. The penBlueCore VM software is responsible for bridging the CPU UART 588interface to USB when the pen is operating in tethered mode. The penBlueCore VM software is not used when the pen is operating in Bluetoothmode.

The pod BlueCore VM software is responsible for sensing when the pod ischarging a pen 400, controlling the pod LEDs appropriately, andcommunicating with the host PC via USB.

For a detailed description of the software modules, reference is made toUS Publication No. 2006/0028459, the contents of which are hereinincorporated by reference.

The present invention has been described with reference to a preferredembodiment and number of specific alternative embodiments. However, itwill be appreciated by those skilled in the relevant fields that anumber of other embodiments, differing from those specificallydescribed, will also fall within the spirit and scope of the presentinvention. Accordingly, it will be understood that the invention is notintended to be limited to the specific embodiments described in thepresent specification, including documents incorporated bycross-reference as appropriate. The scope of the invention is onlylimited by the attached claims.

The invention claimed is:
 1. A method of decoding a coding patterndisposed on or in a substrate, said method comprising the steps of: (a)operatively positioning an optical reader relative to a surface of saidsubstrate; (b) capturing an image of a portion of said coding pattern,said coding pattern comprising a plurality of square tags of length lidentifying two-dimensional location coordinates, each tag comprising: aplurality n₁ of x-coordinate data symbols encoding a respective (n₂, k)Reed-Solomon code for an x-coordinate; a plurality n₁ of y-coordinatedata symbols encoding a respective (n₂, k) Reed-Solomon code for ay-coordinate; a number z of said x-coordinate data symbols positioned ina central column of width q in each tag with a remaining (n₁−z)x-coordinate data symbols distributed evenly on either side of saidcentral column; and a number z of said y-coordinate data symbolspositioned in a central row of said tag of width q with a remaining(n₁−z) y-coordinate data symbols distributed evenly above and below saidcentral row, wherein n₁≧n₂ and z≦1; (c) sampling and decoding at leastone of: x-coordinate data symbols within said imaged portion; andy-coordinate data symbols within said imaged portion, wherein saidimaged portion has a diameter of at least (l+q)√2 and less than (2l)√2and is guaranteed to contain at least (n₁+z)/2 data symbols from each ofsaid Reed-Solomon codes, and wherein one or more symbol errors arecorrectable in each of said codes during said decoding.
 2. The method ofclaim 1, wherein n₁=n₂ and none of said x-coordinate or y-coordinatedata symbols is replicated within each tag.
 3. The method of claim 1,wherein n₂>2k and z≧k.
 4. The method of claim 1, wherein: z is from 1 to253; k is from 1 to 253; n₁ is from 3 to 255; and n₂ is from 3 to 255.5. The method of claim 4, wherein up to 126 symbol errors are correctedduring said decoding.
 6. The method of claim 1, wherein said sampledx-coordinate data symbols and y-coordinate data symbols within saidimaged portion define respective sampled x-coordinate and y-coordinatecodewords, and wherein any missing coordinate data symbols fromrespective (n₂, k) Reed-Solomon codes are treated as erasures in saidsampled codewords.
 7. The method of claim 6, wherein a number of saidmissing coordinate data symbols from respective (n₂, k) Reed-Solomoncodes varies from 0 to (n₁−z)/2 depending on an alignment of said imagedportion with a tag to be decoded.
 8. The method of claim 1, wherein eachdata symbol is encoded using multi-pulse position modulation.
 9. Themethod of claim 8, wherein each data symbol is represented by dmacrodots, each of said d macrodots occupying a respective position froma plurality of predetermined possible positions p, the respectivepositions of said d macrodots representing one of a plurality ofpossible data values, wherein p>d.
 10. The method of claim 9, wherein dis an integer value of 2 to 6 and p is an integer value of 4 to
 12. 11.The method of claim 9, wherein each data symbol provides i possiblesymbol values, and wherein any unused symbol values are treated aserasures.
 12. The method of claim 9, wherein q=2 s, 3 s, 4 s or 5 s, ands is defined as a spacing between adjacent macrodots.
 13. The method ofclaim 1, wherein said coding pattern comprises a plurality of targetelements defining a target grid, said target grid comprising a pluralityof grid cells, wherein neighboring grid cells share target elements andwherein each tag is defined by a plurality of contiguous grid cells. 14.The method of claim 13, wherein each grid cell defines a symbol groupcontaining a plurality of data symbols, wherein each symbol groupcomprises at least one of said x-coordinate symbols or at least one ofsaid y-coordinate symbols.
 15. The method of claim 14, wherein each tagcomprises one or more common Reed-Solomon codewords identifying a regionidentity associated with said surface, each common codeword being commonto a plurality of contiguous tags and each common codeword comprising arespective set of common data symbols, wherein said method comprises thefurther steps of: sampling and decoding common data symbols within saidimaged portion; and determining a region identity using said decodedcommon data symbols.
 16. The method of claim 14, wherein each symbolgroup comprises one or more registration symbols, said registrationsymbols identifying one or more of: a tag encoding format; a translationof a symbol group relative to a tag containing said symbol group; anorientation of a layout of said data symbols with respect to said targetgrid; and a flag, wherein said method comprises the further step of:sampling and decoding registration symbols within the imaged portion;and using the decoded registration symbols to decode the x-coordinatedata symbols and y-coordinate data symbols.
 17. The method of claim 16,wherein said method comprises the further step of: using the decodedregistration symbols to determine a tag encoding format for said codingpattern, said tag encoding format identifying a number of macrodotscontained in each of said data symbols.
 18. The method of claim 16,wherein said method comprises the further step of: using the decodedregistration symbols to determine whether at least one tag within saidimaged portion is flagged or unflagged; and providing feedback to a userin the event that the tag is flagged.
 19. A system which decodes acoding pattern, said system comprising: (A) a substrate having a codingpattern disposed thereon or therein, said coding pattern comprising aplurality of square tags of length l identifying two-dimensionallocation coordinates, each tag comprising: a plurality n₁ ofx-coordinate data symbols encoding a respective (n₂, k) Reed-Solomoncode for an x-coordinate; a plurality n₁ of y-coordinate data symbolsencoding a respective (n₂, k) Reed-Solomon code for a y-coordinate; anumber z of said x-coordinate data symbols positioned in a centralcolumn of width q in each tag with a remaining (n₁−z) x-coordinate datasymbols distributed evenly on either side of said central column; and anumber z of said y-coordinate data symbols positioned in a central rowof said tag of width q with a remaining (n₁−z) y-coordinate data symbolsdistributed evenly above and below said central row, wherein n₁≧n₂ andz≧k; and (B) an optical reader comprising: an image sensor whichcaptures an image of a portion of said coding pattern, said image sensorhaving a field of view with a diameter of at least (l+q)√2 and less than(2l)√2; and a processor configured to the steps of: (i) sampling atleast (n₁+z)/2 data symbols from at least one of said Reed-Solomoncodes; and (ii) decoding at least one of: said x-coordinate data symbolswithin said imaged portion; and said y-coordinate data symbols withinsaid imaged portion, wherein said processor is configured to correct oneor more symbol errors in each of said codes when decoding saidx-coordinate and y-coordinate data symbols.
 20. An optical reader whichdecodes a coding pattern disposed on or in a substrate, said codingpattern comprising a plurality of square tags of length l identifyingtwo-dimensional location coordinates, each tag comprising: a pluralityn₁ of x-coordinate data symbols encoding a respective (n₂, k)Reed-Solomon code for an x-coordinate; a plurality n₁ of y-coordinatedata symbols encoding a respective (n₂, k) Reed-Solomon code for ay-coordinate; a number z of said x-coordinate data symbols positioned ina central column of width q in each tag with a remaining (n₁−z)x-coordinate data symbols distributed evenly on either side of saidcentral column; and a number z of said y-coordinate data symbolspositioned in a central row of said tag of width q with a remaining(n₁−z) y-coordinate data symbols distributed evenly above and below saidcentral row, wherein n₁≧n₂ and z≧k; said optical reader comprising: animage sensor which captures an image of a portion of said codingpattern, said image sensor having a field of view with a diameter of atleast (l+q)√2 and less than (2l)√2; and a processor configured toperform the steps of: (i) sampling at least (n₁+z)/2 data symbols fromat least one of said Reed-Solomon codes; and (ii) decoding at least oneof: said x-coordinate data symbols within said imaged portion; and saidy-coordinate data symbols within said imaged portion, wherein saidprocessor is configured to correct one or more symbol errors in each ofsaid codes when decoding said x-coordinate and y-coordinate datasymbols.